Such closure caps are used, for example, in motor vehicle cooling systems, either directly as a radiator cap, or as a closure for the expansion tank. Here, the closure cap can either be screwed on by means of a screw thread, or it can be pushed on and rotated by means of a bayonet element. In connection with motor vehicle cooling systems there is a problem in regard to the closure caps in that as a rule the pressure is very high because of the high temperature in the cooling system. Even if at the time the motor is turned off the temperature in the cooling system is not too high, there can be a temperature, and therefore a pressure increase in the cooling system because of a certain amount of residual heating after the motor has been turned off. If the closure cap is removed during this time, there is an acute danger of scalding of the respective user. To counteract this, closure caps with a temperature-dependent unscrewing protection, such as described at the outset, have been developed which, in case of an excessive temperature in the cooling system, do not permit the user to remove the closure cap by actuating the cap, since during excess temperature the cap turns without contact with the engagement element.
A closure cap is known, for example, from Published international application WO 95/32904. With closure caps in accordance with this publication, a temperature-dependent control element constituted by a bimetal element is provided in accordance with a variant in a comparatively voluminous hollow chamber within the cap and above the engagement element, and in accordance with another variant a U-shaped coupling element can be lifted or lowered in the axial direction. Although both embodiments operate satisfactorily, the space between the engagement element and the cover required for this is considerable.